The present invention relates to a hydraulic system for a working machine. The invention also relates to a control unit for controlling an implement of a working machine, a method for controlling the implement and a control system for the working machine. The invention is applicable on vehicles, in particularly working machines such as wheel loaders or graders. However, although the invention will mainly be described in relation to a wheel loader, the invention is of course also applicable for hydraulic system used in other applications, such as e.g. excavators or other construction equipments.
Working machines, such as e.g. wheel loaders, often utilize a lift arm provided with an implement, and one or more hydraulic cylinders for raising and lowering the lift arm. The implement can, for example, be a bucket or a plow. These types of implements are often provided with exchangeable wear parts, such as cutting edges and teeth. When using the implement, the wear parts are exposed to large stresses and strains due to the forces that affect the implement, which result in high wear of the components. It is hence a need of controlling the forces acting on the wear parts of the implements.
US 2011/0 220 231 disclose a boom cylinder control circuit for a construction machine. The control circuit comprises a two-way mode in which the boom cylinder can be freely moved up and down by external forces. This is useful when making the ground even by using the bucket.
The solution provided in US 2011/0 220 231 provides a ground contact force between the ground and the bucket which is dependent on the weight of the boom and the bucket. Hence, a heavier boom and bucket will provide more wear on the wear parts in relation to a less heavy boom and bucket.
Accordingly, there is a further need of reducing wear on the tool.
It is a desirable to provide a hydraulic system which enables the wear on the tool to be reduced.
According to a first aspect of the invention, there is provided a hydraulic system for a working machine comprising a hydraulic cylinder for raising and lowering an implement, the hydraulic system comprising a sensor for measuring a pressure in the hydraulic cylinder; and a control unit for controlling the movement of the hydraulic cylinder; wherein the control unit is configured to compare a pressure value measured by means of the sensor with a preset target pressure level range and to control the pressure in the hydraulic cylinder to be within the preset target pressure level range by adding or draining hydraulic fluid to/from a pressure side of the hydraulic cylinder for maintaining a substantially constant ground contact force between the implement and the ground.
The wording “pressure side” should in the following and throughout the entire description be interpreted as a side of the hydraulic cylinder in which hydraulic, fluid can be added and/or drained. More specifically, the pressure side of the hydraulic cylinder is the side of the cylinder in which hydraulic fluid is added or drained in order to control the ground contact force between the implement and the ground and also to control the lifting, lowering or tilting of the implement. Accordingly, adding or draining hydraulic fluid to/from the pressure side of the hydraulic cylinder will adjust the hydraulic pressure on the pressure side such that the ground contact force acting between the implement and the ground is actively controlled. The pressure side of the hydraulic cylinder can hence, as will be described further below, be a piston side of the hydraulic cylinder or a piston rod side of the hydraulic cylinder.
Furthermore, the preset target pressure level range can be set in a number of different manners and can be different depending on the specific application of the implement. For example, the preset target pressure level range can be set by the user of the working machine by means of a human-machine interface (HMI) where the user sets a desired pressure level in dependence on the specific operation he/she is intending to perform by the implement. The preset target pressure level range may also be preset without the need of an operator actively setting the pressure level. The latter alternative may be useful if the working machine is used for a single purpose where the ground contact force between the implement and the ground is always approximately the same. Examples of different pressure levels and their applications will be described further below. Still further, instead of having to manually set a preset target pressure level range, the system may be pre-calibrated with different modes which are linked to specific respective pressure values. Hereby, the user of the working machine may only need to choose what kind of operation he/she will perform and the system then sets the desired target pressure level range. Hence, the user may, for example, choose a “snow-plowing-mode” for which a specific pressure value is preset without the need of the user having to manually input a specific pressure value to the hydraulic system.
Moreover, the implement can be a bucket or the like coupled to a lift arm of the working machine. Hence, the wording “raising and lowering” should also include tilting of the implement, which when tilted is raised or lowered in relation to e.g. the ground by means of a tilt cylinder. The implement can also be a tool connected at e.g. a rear end of the working machine which is in contact with the ground by means of wheels or blades in cases where the working machine is a grader.
The present invention is based on the insight that by actively adding or draining hydraulic fluid to the pressure side of the hydraulic cylinder, the contact force between the implement and the ground can be kept substantially constant. This substantially constant contact force between the implement and the ground can be chosen to be at different levels depending on the specific application of the working machine. Hereby, the present invention provides for different modes of operation where the substantially constant contact force between the implement and the ground is set at different levels depending on the specific mode, which will be described further below. Hence, the operator of the working machine can use the “correct” ground contact force between the implement and the ground during the specific operation.
An advantage of the present invention is that the wear of the implement during use can be reduced by setting the ground contact force between the implement and the ground substantially constant. This is achieved since the adding/chaining of hydraulic fluid will control the raising/lowering of the implement such that the forces on the implement does not increase/decrease to undesired levels during operation.
Furthermore, the fuel consumption of the working machine may also be reduced by the hydraulic system of the present invention, since a relatively continuous operation mode is achieved by keeping the ground contact force substantially constant.
Accordingly, by controlling the ground contact force to be substantially constant, the force can be kept at desired levels. Hereby, the force can be controlled such that it does not damage the ground if such a scenario is desired, or provides a lame force on the ground to e.g. level out the ground if such a scenario is desired.
According to an example embodiment, the sensor may be in fluid communication with the pressure side of the hydraulic cylinder and electrically connected to the control unit.
Hereby, the sensor receives an indication of the pressure level in the hydraulic cylinder, which pressure level is received by the control unit. The control unit then determines whether to add or drain hydraulic fluid to/from the hydraulic cylinder, or, if the pressure level is within the preset target pressure level range to neither add or drain hydraulic fluid to/from the hydraulic cylinder.
According to an example embodiment, the hydraulic system may further comprise a hydraulic accumulator unit arranged in fluid communication with the pressure side of the hydraulic cylinder and the sensor.
An advantage of providing a hydraulic accumulator unit is that the adding/draining of hydraulic fluid to/from the hydraulic cylinder can be made more rapidly in comparison to adding hydraulic fluid from e.g. a pump unit or chain to e.g. a drain tank of the hydraulic system. More specifically, the hydraulic accumulator unit may receive hydraulic fluid from the hydraulic cylinder when the control unit determines that hydraulic fluid should be drained from the hydraulic cylinder. The hydraulic fluid provided to the hydraulic accumulator unit is then kept in the hydraulic accumulator unit until the control unit determines that hydraulic fluid should be added to the hydraulic cylinder. The hydraulic fluid is thereafter added to the hydraulic cylinder from the hydraulic accumulator unit. Hence, the hydraulic accumulator unit may thus serve the purpose of storage for hydraulic fluid. The hydraulic accumulator unit is in constant fluid communication with the pressure side of the hydraulic cylinder when hydraulic fluid is to be added or drained to/from the pressure side of the hydraulic cylinder. When the hydraulic cylinder is drained from hydraulic fluid, the hydraulic fluid is added to the hydraulic accumulator unit, and when hydraulic fluid is to be added to the hydraulic cylinder, the hydraulic fluid, is drained from the hydraulic, accumulator unit and added, to the hydraulic cylinder. Hence, hydraulic fluid can be added or drain to/from the hydraulic cylinder without the need of adding hydraulic fluid from the pump unit or drain hydraulic fluid to the drain tank, as long as the amount of hydraulic fluid added or drained to/from the hydraulic accumulator unit is within a specific range set by the hydraulic accumulator unit.
It should be readily understood that not all of the hydraulic fluid drained from the hydraulic cylinder must be provided to the hydraulic accumulator unit. Some of the hydraulic fluid may also be drained to e.g. a drain tank of the hydraulic system.
Likewise, when adding hydraulic fluid to the hydraulic cylinders, hydraulic fluid may be provided to the hydraulic cylinder from both the hydraulic accumulator unit as well as from e.g. a pump unit of the hydraulic system. This may be the case if the amount of hydraulic fluid added or drained to/from the hydraulic cylinders exceeds the specific range set by the hydraulic accumulator unit. Hereby, further hydraulic fluid may be added from the pump unit or drained to the drain tank.
According to an example embodiment, the pressure side of the hydraulic cylinder may be a piston side of the hydraulic cylinder, wherein hydraulic fluid is added or drained to/from the piston side of the hydraulic cylinder for maintaining the substantial constant ground contact force between the implement and the ground.
Hereby, the hydraulic system can add or drain hydraulic fluid to/from the piston side of the hydraulic cylinder, which is described further below. Adding or draining hydraulic fluid to the piston side of the hydraulic cylinder will in the following and throughout the entire description be denoted as the first operating mode.
An advantage of the first operating mode is that the contact force between the implement and the ground can be controlled to be within the range from zero to a force defined by the dead weight of the lift arm and the weight of the implement. Providing hydraulic fluid to/from the piston side of the hydraulic cylinder in this manner is advantageous for working operations such as e.g. plowing snow, where it is desired to keep the ground contact force between the implement and the ground relatively small in order to reduce wear on the implement as well as on the ground which is being plowed. When adding or draining hydraulic fluid to/from the piston side of the hydraulic cylinder as described above, a piston rod side of the hydraulic cylinder may be connected to a drain tank of the hydraulic system.
According to an example embodiment, the control unit may be configured to control a control valve such that a pump unit adds hydraulic fluid to the piston side of the hydraulic cylinder if the measured pressure value is below the preset pressure level range.
According to an example embodiment, the control unit may be configured to control a control valve such that hydraulic fluid is drained from the piston side of the hydraulic cylinder if the measured pressure value is above the preset pressure level range.
According to an example, the first operating mode works as follows. When the working machine is working on a relatively flat surface, no hydraulic fluid needs to be added or drained to/from the piston side of the hydraulic cylinder since the ground contact force between the implement and the ground will be kept relatively constant and within the preset range. However, if there is a bump on the ground, the hydraulic fluid pressure in the piston side of the hydraulic cylinder will decrease, which is sensed by the sensor, and thus the contact force between the implement and the ground will increase, when the implement is driven over the bump. Hereby, in order to maintain the substantially constant ground contact force between the implement and the ground, hydraulic fluid is added to the piston side of the hydraulic cylinder. The hydraulic fluid can be provided from the pump unit and/or from the above described hydraulic accumulator unit.
On the other hand, if there is a hole in the ground, the hydraulic fluid pressure in the piston side of the hydraulic cylinder will increase, which is sensed by the sensor, and thus the ground contact force between the implement and the ground will be reduced, when the implement is driven over the hole. In order to maintain the substantially constant contact force between the implement and the ground, hydraulic fluid is drained from the piston side of the hydraulic cylinder. The hydraulic fluid can be drained from the hydraulic cylinder to a drain tank of the hydraulic system and/or to the above described hydraulic accumulator unit.
According to an example embodiment, the pressure side of the hydraulic cylinder may be a piston rod side of the hydraulic cylinder, wherein hydraulic fluid is added or drained to/from the piston rod side of the hydraulic cylinder for maintaining the substantial constant ground contact force between the implement and the ground.
Hereby, the hydraulic system can add or drain hydraulic fluid to/from the piston rod side of the hydraulic cylinder, which is described further below. Adding or draining hydraulic fluid to the piston rod side of the hydraulic cylinder will in the following and throughout the entire description be denoted as the second operating mode.
An advantage of the second operating mode is that the ground contact force between the implement and the ground can be controlled to be within the range from a force defined by the dead weight of the lift arm and the implement to a force defined by the dead weight of the lift arm implement and a force from the hydraulic cylinder. Providing hydraulic fluid to/from the piston rod side of the hydraulic cylinder in this manner is advantageous for working operations when it is desired to e.g. pack the ground with a relatively high specific force, which force is larger than the force defined by the dead weight of the lift arm and the implement. According to an example where the working machine is a wheel loader, the largest ground contact force between the implement and the ground is then obtained when the front wheels of the wheel loader is no longer in contact with the ground and thus the wheel loader is only supported by the rear wheels and the implement, i.e. only the rear wheels and the implement of the wheel loader is in contact with the ground. When adding or draining hydraulic fluid to/from the piston rod side of the hydraulic cylinder as described above, the piston side of the hydraulic cylinder may be connected to a drain tank of the hydraulic system.
According to an example embodiment, the control unit may be configured to control a control valve such that a pump unit adds hydraulic fluid to the piston rod side of the hydraulic cylinder if the measured pressure value is below the preset pressure level range.
According to an example embodiment, the control unit may be configured to control a control valve such that hydraulic fluid is drained from the piston rod side of the hydraulic cylinder if the measured pressure value is above the preset pressure level range.
According to an example, the second operating mode works as follows. When the working machine is working on a relatively flat surface, no hydraulic fluid needs to be added or drained to/from the piston rod side of the hydraulic cylinder since the around contact force between the implement and the ground will be kept relatively constant and within the preset range. However, if there is a bump on the ground, the hydraulic fluid pressure in the piston rod side of the hydraulic cylinder will increase, which is sensed by the sensor, and thus the ground contact force between the implement and the around will increase, when the implement is driven over the bump. Hereby, in order to maintain the substantially constant contact force between the implement and the ground, hydraulic fluid is drained from the piston rod side of the hydraulic cylinder. The hydraulic fluid can be drained from the hydraulic cylinder to a drain tank of the hydraulic system and/or to the above described hydraulic accumulator unit.
On the other hand, if there is a hole in the ground, the hydraulic fluid pressure in the piston rod side of the hydraulic cylinder will decrease, which is sensed by the sensor, and thus the ground contact force between the implement and the ground will be reduced, when the implement is driven over the hole. In order to maintain the substantially constant ground contact force between the implement and the ground, hydraulic fluid is added to the piston rod side of the hydraulic cylinder. The hydraulic fluid added to the hydraulic cylinder can be provided from the pump unit and/or from the above described hydraulic accumulator unit.
The above description of the first and second operating modes have been described by the assumption that the implement is raised relative to the ground when the piston of the hydraulic cylinder is moved in a direction from the piston side to the piston rod side. However, the invention should not be limited to this assumption and hence works equally as well if the orientation of the hydraulic cylinder for some reason is the opposite, i.e. that the implement is raised relative to the ground when the piston of the hydraulic cylinder is moved in a direction from the piston rod side to the piston side. In such a case, the above examples of adding/draining hydraulic fluid to/from the piston side or the piston rod side are executed oppositely to the above description. More precisely, hydraulic fluid is drained from the piston side of the hydraulic cylinder if the implement is driven over a bump in the first operating mode, and added to the piston side if the implement is driven over a hole in the first operating mode. The same applies for the second operating mode and thus needs no further explanation.
According to an example embodiment, a piston side sensor valve may be arranged in fluid communication with the piston side of the hydraulic cylinder and the sensor. Hereby, when it is desired to measure the pressure in the piston side of the hydraulic cylinder, the piston side sensor valve can be positioned in an opened state.
According to an example embodiment, a piston rod side drain valve may be arranged in fluid communication with the piston rod side of the hydraulic cylinder and a drain tank.
According to an example embodiment, the control unit may be configured to position the piston side sensor valve and the piston rod side drain valve in an open state when adding or draining hydraulic fluid to/from the piston side of the hydraulic cylinder. Hereby, the hydraulic system may be operated in the first operating mode.
According to an example embodiment, a piston rod side sensor valve may be arranged in fluid communication with the piston rod side of the hydraulic cylinder and the sensor. Hereby, when it is desired to measure the pressure in the piston rod side of the hydraulic cylinder, the piston rod side sensor valve can be positioned in an opened state.
According to an example embodiment, a piston side drain valve may be arranged in fluid communication with the piston side of the hydraulic cylinder and a drain tank.
According to an example embodiment, the control unit may be configured to position the piston rod side sensor valve and the piston side drain valve in an open state when adding or draining hydraulic fluid to/from the piston rod side of the hydraulic cylinder. Hereby, the hydraulic system may be operated in the second operating mode.
According to a second aspect of the present invention, there is provided a control unit for controlling an implement of a working machine comprising a hydraulic cylinder for raising and lowering the implement, and a sensor for measuring a pressure in the hydraulic cylinder; wherein the control unit is configured to compare a pressure value measured by means of the sensor with a preset target pressure level range and to control the pressure in the hydraulic cylinder to be within the preset target pressure level range by adding or draining hydraulic fluid to/from a pressure side of the hydraulic cylinder for maintaining a substantially constant ground contact force between the implement and the ground.
Effects and features of this second aspect of the present invention are largely analogous to those described above in relation to the first aspect of the present invention.
According to a third aspect of the present invention, there is provided a method for controlling an implement of a working machine comprising a hydraulic cylinder for raising and lowering the implement; wherein the method comprises the steps of comparing a pressure value in the hydraulic cylinder with a preset target pressure level range; and controlling the pressure in the hydraulic cylinder to be within the preset target pressure level range by adding or draining hydraulic fluid to/from a pressure side of the hydraulic cylinder for maintaining a substantially constant ground contact force between the implement and the ground.
Effects and features of this third aspect of the present invention are largely analogous to those described above in relation to the first and second aspects of the present invention.
According to a fourth aspect of the present invention, there is provided a control system for a working machine provided with an implement and a hydraulic cylinder for raising and lowering the implement, wherein the control system has a selectable mode for controlling the lift arm, in which selectable mode hydraulic fluid is added or drained to/from a pressure side of the hydraulic cylinder for maintaining a substantially constant ground contact force between the implement and the ground.
An advantage is that the control system may utilize different modes depending on the specific working operation of the working machine. Hence, a control system is provided which is easy to handle by the operator of the working machine.
According to an example embodiment, the selectable mode may have a first operating mode in which the substantially constant ground contact force being lower than a ground contact force caused by the dead weight of the lift arm and the implement when the implement passively rest against the ground.
According to an example embodiment, hydraulic fluid may be configured to be added or drained to/from a piston side of the hydraulic cylinder when the control system is in the first operating mode.
According to an example embodiment, the selectable mode may have a second operating mode in which the substantially constant ground contact force being higher than a ground contact force caused by the dead weight of the lift arm and the implement when the implement passively rest against the ground.
According to an example embodiment, hydraulic fluid may be configured to be added or drained to/from a piston rod side of the hydraulic cylinder when the control system is in the second operating mode.
The first and second operating modes are described in detail above in relation to the first aspect of the present invention.
According to an example embodiment, the control system may be further configured to switch from the first operating, mode to the second operating mode by means of moving a piston rod side drain valve from an open position to a closed position and moving a piston side drain valve from a closed position to an air off position, wherein the piston rod side drain valve is arranged in fluid communication with the piston rod side of the hydraulic cylinder and a drain tank and the piston side drain valve is arranged in fluid communication with the piston side of the hydraulic cylinder and the drain tank.
Hereby, an easy and convenient way of switching between the first and second operating modes is provided, which is especially advantageous in situations where it is desired to have a relatively low contact force between the implement and the ground at a first instance and in a second instance have a relatively large contact force between the implement and the ground. An example of such a situation can be when the working machine in the first instance is plowing snow from the ground and in the second instance packing the ground which was previously covered by the snow.
Further effects and features of this fourth aspect of the present invention are largely analogous to those described above in relation to the first, second and third aspects of the present invention.
Further features of, and advantages with, the present invention will become apparent when studying the appended claims and the following description. The skilled person realize that different features of the present invention may be combined to create embodiments other than those described in the following, without departing from the scope of the present invention.